Can I import a Golang package based on the OS I'm building for?

Say I have a go project that based on which OS, and in some cases which distro, I want to use say a Systemd client package vs an Upstart client package vs a sysv client package vs a launchd client package. Is it possible to selectively import each package so I only import the one I need per OS/distro I'm building for? Or do I have to import each package for each OS/distro?

> Package build
>
> Build Constraints
>
> A build constraint, also known as a build tag, is a line comment that
> begins
>
> // +build
>
> that lists the conditions under which a file should be included in the
> package. Constraints may appear in any kind of source file (not just
> Go), but they must appear near the top of the file, preceded only by
> blank lines and other line comments. These rules mean that in Go files
> a build constraint must appear before the package clause.
>
> To distinguish build constraints from package documentation, a series
> of build constraints must be followed by a blank line.
>
> A build constraint is evaluated as the OR of space-separated options;
> each option evaluates as the AND of its comma-separated terms; and
> each term is an alphanumeric word or, preceded by !, its negation.
> That is, the build constraint:
>
> // +build linux,386 darwin,!cgo
>
> corresponds to the boolean formula:
>
> (linux AND 386) OR (darwin AND (NOT cgo))
>
> A file may have multiple build constraints. The overall constraint is
> the AND of the individual constraints. That is, the build constraints:
>
> // +build linux darwin
> // +build 386
>
> corresponds to the boolean formula:
>
> (linux OR darwin) AND 386
>
> During a particular build, the following words are satisfied:
>
> - the target operating system, as spelled by runtime.GOOS
> - the target architecture, as spelled by runtime.GOARCH
> - the compiler being used, either "gc" or "gccgo"
> - "cgo", if ctxt.CgoEnabled is true
> - "go1.1", from Go version 1.1 onward
> - "go1.2", from Go version 1.2 onward
> - "go1.3", from Go version 1.3 onward
> - "go1.4", from Go version 1.4 onward
> - "go1.5", from Go version 1.5 onward
> - "go1.6", from Go version 1.6 onward
> - any additional words listed in ctxt.BuildTags
>
> If a file's name, after stripping the extension and a possible _test
> suffix, matches any of the following patterns:
>
> *_GOOS
> *_GOARCH
> *_GOOS_GOARCH
>
> (example: source_windows_amd64.go) where GOOS and GOARCH represent any
> known operating system and architecture values respectively, then the
> file is considered to have an implicit build constraint requiring
> those terms (in addition to any explicit constraints in the file).
>
> To keep a file from being considered for the build:
>
> // +build ignore
>
> (any other unsatisfied word will work as well, but “ignore” is
> conventional.)
>
> To build a file only when using cgo, and only on Linux and OS X:
>
> // +build linux,cgo darwin,cgo
>
> Such a file is usually paired with another file implementing the
> default functionality for other systems, which in this case would
> carry the constraint:
>
> // +build !linux,!darwin !cgo
>
> Naming a file dns_windows.go will cause it to be included only when
> building the package for Windows; similarly, math_386.s will be
> included only when building the package for 32-bit x86.
>
> Using GOOS=android matches build tags and files as for GOOS=linux in
> addition to android tags and files.

Use build constraints.

Use a single package with multiple files. Each file specializes for a particular OS, architecture, etc, combination.

> A build constraint is evaluated as the OR of space-separated options; each option evaluates as the AND of its comma-separated terms; and each term is an alphanumeric word or, preceded by !

Actually, that will change with Go 1.17 (Q3 2021)

> ## //go:build lines
>
> The go command now understands //go:build lines and prefers them over // +build lines.
>
> The new syntax uses boolean expressions, just like Go, and should be less error-prone.
>
> As of this release, the new syntax is fully supported, and all Go files should be updated to have both forms with the same meaning.
To aid in migration, gofmt now automatically synchronizes the two forms.
For more details on the syntax and migration plan, see https://golang.org/design/draft-gobuild.

From the design document:

> The core idea of the design is to replace the current // +build lines for build tag selection with new //go:build lines that use more familiar boolean expressions.
>
> For example, the old syntax
>
>go
>// +build linux
>// +build 386
>
>would be replaced by the new syntax
>go
>//go:build linux && 386
>
1: https://twitter.com/golang/status/1403016905179422723